1. Field of the Invention
The present invention relates to a powder magnetic core and to a manufacturing method for making the powder magnetic core. The powder magnetic core is used for a transformer or a reactor in a switching power supply.
2. Background of the Related Art
Recently, the sizes and weights of various electronic instruments have been reduced. In association with this trend, the switching power supplies mounted on the electronic instruments have been required to be reduced in size. There has been a particularly strong requirement to reduce the dimensions and thickness of the switching power supplies on notebook-type personal computers, small portable instruments, thin CRTs, flat panel displays and such instruments. However, the transformers, reactors and such main magnetic component parts of conventional switching power supplies occupy large spaces and this puts limits on efforts to reduce the dimensions and thicknesses of the conventional power supplies. Therefore, it is difficult to reduce the volume of the switching power supplies when the volume of transformers, reactors and such main magnetic component parts remain unreduced.
Metal magnetic materials, such as Sendust, Permalloy, and oxide magnetic materials such as ferrite, have been used for transformers, reactors and such main magnetic component parts in the switching power supplies. Although the metal magnetic materials exhibit a high saturation magnetic flux density and high magnetic permeability generally, the metal magnetic materials cause a large eddy current loss in a high frequency range due to the low electrical resistivity thereof. According to recent technical trends, the size of the magnetic component parts are reduced by lowering the relevant inductance values by means of driving the power supply circuits at a high frequency. However, the metal magnetic materials are still unemployable at a high frequency due to the adverse effects of eddy current loss.
The eddy current loss caused by oxide magnetic materials at a high frequency range is low due to the high electrical resistivity thereof. However, it has been impossible to reduce the volume of a component part made of the oxide magnetic material, since the oxide magnetic material is liable to saturate magnetically due the low saturation magnetic flux density thereof. In any case, the magnetic core volume is the most decisive factor that determines the inductance value. Therefore, it is difficult to reduce the dimensions and thicknesses of the magnetic component parts without working to improve the magnetic properties of the magnetic materials.
Since there exist certain limitations on down-sizing the conventional magnetic component parts as described above, the conventional magnetic component parts have been unable to meet the requirements of reducing the dimensions and thicknesses of electronic instruments.
For obviating the problems described above, a high-density sintered magnetic compact that includes metal magnetic particles, 1 to 10 μm in thickness and covered with a metal oxide magnetic material having a spinel composition described by M-FexO4 (here, M=Ni, Mn and Zn, x≦2), has been proposed (see Unexamined Japanese Patent Application Publication No. Sho. 56 (1981)-38402).
International Publication No. 03/015109 proposes a composite magnetic material formed by compression-molding ferromagnetic metal powders having small particle diameters or by compression-molding ferromagnetic intermetallic compound powders having small particle diameters. The ferromagnetic metal powders are covered with respective ferrite coating layers formed by ultrasonic-wave-excited ferrite plating. Magnetic paths are formed between the ferromagnetic metal powders through the ferrite coating layers. The ferromagnetic intermetallic compound powders are covered with respective ferrite coating layers formed by ultrasonic-wave-excited ferrite plating. Magnetic paths are formed between the ferromagnetic intermetallic compound powders through the ferrite coating layers.
In Unexamined Japanese Patent Application Publication No. 2001-85211, for obtaining a soft magnetic compact that exhibits a high density and high specific resistance, a soft magnetic particle including a soft magnetic metal particle, a very resistive material layer (hereinafter referred to as a “very resistive layer”) covering the soft magnetic metal particle, and a chemically-formed phosphate coating film covering the very resistive layer are used.
Recently, a magnetic material for improving the resistivity, which is the weak point of the metal magnetic material, has been proposed. The magnetic material is formed by coating a nonmagnetic insulator oxide coating film (hereinafter referred to as a “nonmagnetic insulator coating”) exhibiting a high electrical resistivity and covering a soft magnetic metal particle exhibiting a high saturation magnetic flux density and a high magnetic permeability. This magnetic material, which utilizes the favorable effects of the nonmagnetic insulator coating for improving the electrical resistivity thereof, facilitates suppressing the eddy current. In other words, the magnetic material can be used in the MHz band and such a high frequency range.
For further reducing the eddy current loss in the MHz band caused in the soft magnetic compact obtained by molding the magnetic material particles described above, it is necessary to thicken the nonmagnetic insulator coating or the very resistive layer formed on the metal particle in order to improve the resistivity of the soft magnetic compact. For example, the resistivity of the soft magnetic compact according to the prior art disclosed in Unexamined Japanese Patent Application Publication No. 2001-85211 and described in Table 3 thereof is improved as compared with that of the comparative example but still insufficiently. Unexamined Japanese Patent Application Publication No. 2001-85211 discloses the volume iron loss of the soft magnetic compact at a frequency as high as 10 kHz. For making the soft magnetic compact work at 1 MHz, it is necessary to further thicken the very resistive layer in order to raise the compact resistivity. However, thickening the nonmagnetic insulator coating or the very resistive layer widens the gap between the metal particles, lowering the magnetic permeability. If the magnetic permeability is improved by thinning the nonmagnetic insulator coating or by thermally treating the soft magnetic compact, formed by press molding, at an elevated temperature, the eddy current loss in the MHz band will increase due to the resistivity lowering.
For further reducing the eddy current loss in the MHz band, Unexamined Japanese Patent Application Publication No. Hei. 11 (1999)-74140 discloses a manufacturing method that thins the thickness of the powder magnetic cores formed by press molding and laminates the thin powder magnetic cores and insulator layers alternately.
Unexamined Japanese Patent Application Publication No. 2000-54083 and Unexamined Japanese Patent Application Publication No. Hei. 9 (1997)-74016 propose a method of manufacturing a multilayered soft magnetic compact that forms a soft magnetic laminate by laminating soft magnetic films and insulator films alternately.
The method disclosed in Unexamined Japanese Patent Application Publication No. Hei. 11 (1999)-74140 laminates two rings, each 5.5 mm in thickness, by hot pressing to form a laminate of 10 mm in thickness. If the total electronic part thickness is thinner than 0.6 mm, the thickness of the layer to be laminated inevitably will be thinner than half the total thickness, e.g., 0.2 mm or thinner. For securing a sufficient mechanical strength, it is difficult to form such a thin core by press molding. The difficulty increases especially with an increasing core area. Since the total thickness is thin, it is necessary for the laminating method, which laminates thin core layers and insulator layers alternately, to control the thickness of each insulator layer to be 0.05 μm or thinner. However, it is substantially difficult to manufacture such a thin core plate by press-molding.
Unexamined Japanese Patent Application Publication No. 2000-54083 and Unexamined Japanese Patent Application Publication No. Hei. 9 (1997)-74016 describe laminate structures, each including magnetic films and insulator films, used for an inductor or a transformer core. Since uniform magnetic layers for the laminate structures are formed by sputtering or by vacuum deposition, it takes a long time to obtain a laminate of 1 to 10 μm in thickness.
In view of the foregoing, it would be desirable to obviate the problems described above. Also, it would be desirable to provide a powder magnetic core that facilitates improving the high-frequency characteristics thereof and reducing the eddy current loss thereof. It would be further desirable to provide a method for manufacturing a laminate structure laminating thin core layers and insulator layers alternately for improving the high-frequency characteristics of the powder magnetic core and for reducing the eddy current loss thereof.